A new approach allows the realization of an electro-optical modulator or a transistor that is integrated on a silicon chip. The operation principle includes the realization of optical multimode interference (MMI) as the operation core of an electro-optical metal-oxide-semiconductor (MOS) field effect transistor (FET). Externally applied voltage generates a depletion region as well as layer of inversion which is full with free carriers. The free carriers vary the relative phase shift as well as the attenuation of one arm of the interferometer due to the plasma dispersion effect in silicon. This variation allows breaking the balance of the mode conversion and results with amplitude modulation in the output of the MMI region. The device does not have high finesse resonance and yet its overall length is only about 18 µm.